Electric utilities employ nuclear plants, hydroelectric plants, fossil fuel steam generation plants, and the like, to meet their base load requirements, i.e., to provide the bulk of the power that is produced to meet the day-to-day power demands of their customers. However, electric utilities also must have the capability of meeting peaking demands, e.g., for summer peaking utilities, the demand created during the hottest part of summer days. Also, electric utilities that participate in so-called "power pools" must meet reserve capacity requirements under which the utility must be able to bring a prescribed amount of additional capacity on line rapidly, for example, within ten minutes of notification. Thus, to meet peaking demands and power pool reserve capacity requirements, electric utilities need to have on hand fast starting, peaking generators, often powered by gas turbines.
Any process or mechanism that will increase the capacity of existing peaking gas turbines at a cost less than that of new turbines will have a significant impact on the cost assumed by electric utilities in meeting their peak demand and power pool capacity credit requirements. One such approach has been to treat the turbine inlet air, as explained in the text below.
The above-mentioned fast starting, peaking combustion turbine systems (utilizing natural gas or fuel oil) operate under the Brayton cycle wherein the inlet air is compressed in a compressor, heated in a combustor and expanded in a turbine whose shaft drives a generator. Throughout this specification when either the term "turbine" or the term "turbine system" is used alone, it will usually refer generally to the entire combustion turbine system including a compressor, combustor and turbine generator rather than the specific expansion turbine component of the system.
It is well known that combustion turbines driving grid synchronized generators operate at constant speed and are essentially "constant volume machines"; i.e., they operate at a fixed volume flow rate of inlet air. It is also known that the generation capacity of a combustion turbine system is roughly proportional to the mass flow rate of the inlet air to the combustion turbine system. Thus, at high ambient atmospheric temperature, the inlet air is less dense resulting in lower generator capacity. A cruel irony is that for many electric utilities, particularly summer peaking utilities, peak demand occurs during the hottest weather conditions when the operational capacity and efficiency of the turbines are at their lowest due to the relatively low density of the inlet air. Thus, utilities that experience peak load during summer months can increase their peak generating capacity by cooling inlet air for combustion turbines if such cooling can be done at a cost less than the construction of additional turbines.
Attempts have been made to cool the inlet air on hot days prior to introducing the air to the turbine system. These cooling methods include evaporative cooling, direct acting cooling and, most recently, the use of cooling water from an ice harvesting system in conjunction with a hydronic coil heat exchanger. Evaporative cooling is limited to the local wet bulb and hydronic coils using water from ice storage have practical design limits of about 40.degree. F. unless brines or antifreeze solutions are used. However, regardless of the cooling mechanism employed, there appears to be a practical lower temperature limit ("icing" limit) specific to the compressor design to which the inlet air may be cooled without dehumidification. This limit exists because the inlet air contains moisture which will actually freeze on the inlet guide vanes and other metal surfaces at inlet air temperatures below this practical lower temperature limit. In theory, this freezing occurs because as the air entering the compressor accelerates to a velocity on the order of 0.5 Mach, the static temperature of the inlet air is depressed to a freezing level. Thus, the density boost and resulting operating advantages to be achieved by just cooling the inlet air appear to be limited.
The density of the inlet air also has been increased by supercharging the air utilizing fans and/or blowers to compress the air by some amount, e.g., up to about 160" H.sub.2 O. However, this approach is also limited in its application because its power consumption is parasitic to the turbine thereby reducing net output.
It has also been proposed in U.S. Pat. No. 3,796,045 to combine the cooling and supercharging approaches mentioned above to achieve a so-called "superchilling" of the inlet air for base load gas turbine systems. The inlet air is first supercharged with a fan or blower to compress the inlet air to a pressure level moderately greater than the atmospheric pressure and thereafter chilled by use of a waste heat recovery, combined cycle system powering a refrigeration cycle.
Finally, in a somewhat related area, the prior art proposes massive hydraulic compression systems, as exemplified by the disclosure of U.S. Pat. No. 5,099,648, which provide the entire compression of the inlet air prior to its heating by the combustor, thereby eliminating the mechanical compressor altogether. These systems utilize entrained air within water columns that descend to tremendous depths, e.g., 1,000 feet, in order to achieve a pressure boost on the order of 10 atmospheres. While these proposals deal with combustion turbines, they are fundamentally different in mechanism and are not applicable to inlet density boost for peaking combustion turbines.
Thus, there continues to be an acute need in the electric utility industry for a practical, commercially sensible way to dramatically lower costs associated with the provision of peaking capacity. While one approach is simply to build and install more and more gas turbines, the approach of improving gas turbine operational capacity and efficiency, particularly in hot weather conditions, appears to offer the best hope for substantial monetary savings at the margin. Any such system needs to be reliable over a long-term operating life and provide a significant increase in capacity at a fraction of the cost of new gas turbines. Furthermore, and importantly, any such system should not have a negative impact on the fast starting capability of the gas turbine. In fact, enhancing fast starting capability is desirable. Additionally, if possible, the system should provide other operational advantages, for example, enhanced load following capabilities that could be achieved by higher turn down ratios and better means to modulate the output of the peaking turbines used by an electric utility.